Vietnamese is a member of Southeast Asian languages. It is written from left to right and based on Latin script although it contains some Vietnamese special characters. In Vietnamese, characters can appear both in upper and lower cases. Like Chinese and Thai, Vietnamese is a tonal language. This means that the way a word is pronounced determines its meaning.
In the Vietnamese language, there are seventeen (BCDGHKLMNPQRSTVX) consonants in both upper and lower cases, resulting in thirty-four consonants. Among these consonants, thirty-two can be found in the regular English alphabet, and two exceptions are and  (D and d with stroke). The Vietnamese alphabet includes twelve vowels, six of which are common to western languages: a, e, i, o, u and y. The remaining six are represented by combining diacritics into regular vowels: â (a with circumflex), {hacek over (a)} (a with breve), ê (e with circumflex), ô (o with circumflex),  (o with horn) and  (u with horn), as shown in FIG. 1.
Furthermore, in the Vietnamese language, there are six different tones. Five of the tones, excluding the neutral tone, are marked with tone marks, occurring with any one of the twelve vowels. Symbolically, the six different tones can be referred to as neutral, acute accent, tilde, grave accent, hook above and dot below. FIG. 2 shows the tonal variations for the word “ga”.
Vowels can be used in combination. Some examples are shown in FIG. 3. When vowels are used in a combined form, only one tonal mark is required to change the tone. For example, there are two or more vowels in the following word: “hai”, “ngeo”, “tieu”, “thuy”, “nguyen”, but only one tonal mark is used in each word, as shown in FIG. 4.
Each of the twelve vowels can occur in any of the six tones. Accordingly, the combination of vowels with tonal marks results in 72 vowels, both in upper and lower case. The overall number of vowels in Vietnamese, including the tonal variations, is 144, as shown in FIG. 5.
In a regular telephone keypad, as shown in FIG. 6, usually eight keys are used for entering alphabetic characters. As shown in FIG. 6, the telephone set 1 has a keypad 10, only keys 102, 103, 104, 105, 106, 107, 108 and 109 are used for entering alphabetic characters:                2ABC, 3DEF, 4GHI, 5JKL, 6MNO, 7PQRS, 8TUV, 9WXYZIn addition, as shown in FIG. 1, the keypad 10 has at least four other keys 101, 110, 111 and 112:        1, *, 0, #As shown in FIG. 1, the telephone 1 also has a display 4 operatively connected to the keypad 10 for displaying the entered alphabetic characters.        
Even if a message is entered through a phone with either upper case or lower case, there are not enough keys to enter a message in Vietnamese, mainly because of the 144 vowels.
Thus, it is advantageous and desirable to provide a method for entering a message or information in Vietnamese in a telephone using the keys on the telephone keypad.